In manufacturing sintered magnets, there has so far been adopted a method including filling a mold with alloy powder as a raw material (filling process), applying a magnetic field to the raw material alloy powder in the mold to orient particles of the raw material alloy powder (orienting process), applying pressure to the oriented raw material alloy powder to make a compression molded article (compression molding process), and performing sintering by heating the compression molded article after releasing the applied pressure (sintering process). Alternatively, there has been adopted a method in which, after the filling process, the orienting process and the compression molding process are carried out at the same time by applying pressure by the use of a press machine while applying a magnetic field to alloy powder as a raw material. At any rate, these methods each perform compression molding by the use of a press machine, and they are thus referred to as “press methods” in this specification.
In contrast to the press methods, there have been developed methods of performing, after filling a mold with alloy powder as a raw material, orientation and sintering of the alloy powder in a state of being held in the mold without carrying out compression molding, thereby manufacturing sintered magnets (see Patent Documents 1 and 2). Such methods as to manufacture sintered magnets without performing a compression molding process are referred to as “PLP (Press-less Process) methods” in this specification. In such a PLP method, in the filling process of filling a mold with alloy powder as a raw material, the raw material alloy powder may be pushed into the mold with a pressure (about 2 MPa or below) sufficiently lower than a pressure applied to the alloy powder during compression molding (several tens MPa in an ordinary case).
Such a PLP method has, in the main, two advantages described below. A first advantage of the PLP method is in that the manufactured sintered magnets have excellent magnetic properties, notably high coercive force. It is known that the smaller the crystalline particles in a sintered magnet, the higher coercive force the sintered magnet can exhibit. In order to achieve a higher coercive force, it is therefore necessary to make the size of alloy powder as small as possible at the preparation stage of alloy powder as a raw material. Then, the surface area of the alloy powder particles as a raw material becomes large; as a result, the particles become vulnerable to oxidation. When magnet alloys undergo oxidation, the coercive force and other magnetic properties thereof may rather undergo deterioration, or the magnet alloys may cause spontaneous ignition in the air. It is therefore desirable that the magnet alloys be treated in a low-oxygen atmosphere. In regard to this point, PLP methods make it possible for facilities to have a smaller size than that in press methods because they require no press machine, and hence the facilities in their entirety become easier to place in a low-oxygen atmosphere. In any PLP method, finely pulverized alloy powder as a raw material can therefore be treated while being prevented from suffering oxidation, and therefore, sintered magnets of high coercive force can be obtained by using such the fine alloy powder.
A second advantage of PLP methods consists in that they can provide sintered magnets of shapes close to those of final products without carrying out machining. In press methods, on the other hand, it is necessary to subject the alloy powder as a raw material to press molding, and the shape of sintered magnets obtained at the stage of having undergone the sintering process is limited to a shape having two parallel planes corresponding to a pair of punches in a press machine. In order to manufacture sintered magnets having shapes other than the foregoing shape, the sintered articles obtained in the press method must be subjected to machining. In contrast to this, sintered articles obtained at the stage of having undergone the sintering process in a PLP method come to have almost the same shape as the cavity of a mold used (which is referred to as “near net shape”) (see Patent Document 1). Accordingly, it becomes possible to obtain sintered magnets of intended shape by adjusting in advance the shape of mold's cavities to the shape of the final products, without carrying out machining.
Because sintered articles generally have undergone shrinkage during the sintering, the sintered articles (and sintered magnets) after sintering are smaller in size than the mold's cavities. At the time when the sintering shrinkage occurs, friction arises between the sintered article and the mold. Accordingly, Patent Document 2 has used a carbon material with a small friction against sintered articles for at least part of the mold, notably as a material for the floor plate. In Patent Document 2, for example, there is a description such that a mold constituted of a stainless steel body having a cavity in the shape of a cuboid and a lid made of a carbon fiber-reinforced carbon composite (C/C composite) is prepared, the cavity is filled with alloy powder as a raw material, the lid is put on the mold, then the orienting process is carried out, further the mold is turned upside down, and thereby the lid made of the carbon material is utilized as the floor plate of the mold. According to such a method, since the carbon fiber-reinforced carbon composite, which is a special and high-priced material, is used only for the lid, cost savings can be made.
Since PLP methods each have the foregoing two advantages, the sintered magnets manufactured in accordance with them can be used suitably for the rotors and stators of motors in particular. An explanation for the case of using a sintered magnet for the rotor (the case in which the stator is an electromagnet) is given below. Likewise, the case of using a sintered magnet for the stator (the case in which the rotor is an electromagnet) can be explained.
During the rotation of a motor, the rotor moves in an external magnetic field generated by the stator, and thereby the direction of the external magnetic field applied to a magnet of the rotor varies drastically. In such a circumstance, a sintered magnet used for the rotor has to maintain magnetization against the external magnetic field, and therefore is required to have high coercive force, which is an indicator of such a capability. In addition, the rotor in use undergoes a temperature rise from room temperature to about 200° C. in the case of a car's motor, and hence sintered magnets having high coercive force over all of such a temperature range have been required. By virtue of the first advantage of PLP methods, sintered magnets having such a high coercive force can be made suitably in accordance with the PLP methods.
In addition, as shown in, for example, Patent Document 3, the rotor is generally used in a shape that two or more sintered magnets each having a front surface which is partially cylindrical in shape are combined together so as to make the front surface of the rotor in its entirety into a cylindrical face. A back surface (a surface opposed to the front surface) of each sintered magnet is, though may be a partially cylindrical face similarly to the front surface, planar in shape in Patent Document 3, and the rotor in its entirety has a convex shape, that is, it is thick in the vicinity of a center in its rotational direction and thin in the vicinities of both ends thereof. To this sintered magnet is applied a magnetic field in the thickness direction during the orienting process, and thereby magnetization is imparted in the thickness direction of the sintered magnet. By virtue of the second advantage of a PLP method, the sintered magnet in such a shape can be made suitably through the use of a mold constituted of a main body having a cavity convex in a downward direction and a lid having a flat face to be pressed and in accordance with the PLP method.
Patent Document 1: JP-A-2006-019521
Patent Document 2: JP-A-2009-049202
Patent Document 3: JP-A-2015-050880